validate_chain_list() only compares colnames of chains 1 and 2, ignoring chains 3+

[utkarshpawade]

In validate_chain_list() (R/helpers-mcmc.R), the colnames consistency check uses identical(cnames[, 1], cnames[, 2]), which only compares the first two chains. If chain 3 or later has different parameter names/order, the validation silently passes.